1. Field of the Invention
The present invention relates to an image processing apparatus having a function of restraining people from copying a specific original document.
2. Description of the Prior Art
With recent development of color image forming apparatuses each having a device such as a CCD (solid-state imaging device) and digital color copying apparatuses such as laser beam printers or ink jet printers, reliable reproduction of multicolor original documents has been achieved, including elements of color gradation.
However, the aforesaid conventional example encounters the problem of the danger of forgery of paper sheets or documents such as money paper, the copying of which must be inhibited. Since each of the aforesaid apparatuses has no function designed to inhibit such copying of specific original documents such as paper money, a serious problem can be foreseen if such specific original documents are copied.
As a countermeasure against such forgery, a variety of systems for use in copying machines or the like for recognizing the specific original document have been suggested. provided will now be described about a system such as an electrophotographic color copying machine for forming an image according to a so-called frame sequential method. In a case where an image is formed according to the frame sequential method, an original document for, for example, one image plane is read. The read data is color-decomposed, and, first, a cyan (C) component for one image plane is copied onto copying paper. Then, the same original document is again read for one image plane and, as the ensuing component, a magenta (M) component is copied, being superposed on the same copying paper. Furthermore, the aforesaid operations are repeated sequentially for a yellow (Y) component and a black (BK) component. As a result, a full color image can be formed. Therefore, if the detection of the specific original document is performed during the operation of reading the original document for three image planes and an image such as a pattern which informs a viewer that the copied paper is a forged paper is printed at the time of performing the last printing operation in black, forgery can be prevented.
However, in a so-called dot sequential type image forming apparatus in which a full color image is formed in the order of the pixels by printing images of four colors for each pixel, a portion of the specific original document has already, and undesirably, been copied by the time the subject original document is determined to be the specific original document. For example, if it has been detected that the subject original document is the specific original document at the time when about one-third of the original document has been copied, the forger may be able to use the copied portion of the image by veiling a portion. Therefore, forgery cannot be prevented perfectly.